Hypercompact H II regions in the Galactic Plane

Our method combines four interferometric radio continuum surveys (THOR, CORNISH, MAGPIS, and White2005) with far-infrared and sub-mm Galactic Plane surveys to identify embedded HII regions with positive spectral indices; 120 positive spectrum HII regions have been identified from a total sample of 534 positive spectral index radio sources.

None of these HII regions, including the known HC HII regions recovered in our search, fulfills the canonical definition of an HC HII region at 5GHz. We suggest that the current canonical definition of HC HII regions is not accurate and should be revised to include a hierarchical structure of ionized gas that results in an extended morphology at 5GHz. Correlating our search with known ultracompact (UC) HII region surveys, we find that roughly half of detected UC HII regions have positive spectral indices, instead of more commonly assumed flat and optically thin spectra.